Conventionally, there are known devices that vary a difference in repetition frequency between two lasers by vertically fluctuating a repetition frequency of a slave laser, while keeping a repetition frequency of a master laser constant (see, for example, Patent Document 1 (Patent Publication No. 4786767)).
In this device, the slave laser includes a piezoelectric element. The device is designed to add a component vertically fluctuating (for example, a sinusoidal voltage) to a voltage which is to be applied to a piezo driver for driving the piezoelectric element (see, for example, FIGS. 1 and 3 of Patent Document 1).